Methods and apparatuses for signaling in dynamic time division duplex systems

ABSTRACT

The present invention relates to a method in a UE served by a network node, a method in the network node, the UE, and the network node. The network node is applying dynamic TDD with flexible subframes. The method comprises receiving a first configuration message from the network node indicating a TDD reference configuration, and determining in which subframe to signal HARQ information based on the TDD reference configuration. The method further comprises receiving a second configuration message from the network node indicating a set of DL subframes that may comprise explicit signaling messages, monitoring the indicated set of DL subframes, and receiving an explicit signaling message in response to monitoring. The explicit signaling message designates a subframe in which the UE shall receive a DL signal. The method also comprises preparing to receive the DL signal in the designated subframe.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/888,980, filed Nov. 4, 2015, which was the National Stage ofInternational Application No. PCT/SE2014/050574, filed May 9, 2014,which claims the benefit of U.S. Provisional Application No. 61/821,800,filed May 10, 2013, the disclosures of each of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present application relates generally to signaling in a dynamic TimeDivision Duplex (TDD) system where at least one subframe is a flexiblesubframe assigned as either a downlink subframe or an uplink subframeand, more specifically, to a network node, a user equipment, and methodsin the network node and the user equipment for enabling the userequipment to determine a subframe in which to receive a downlink signaland a subframe in which to signal Hybrid Automatic Repeat Request (HARQ)information.

BACKGROUND

3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) isthe fourth-generation mobile communication technologies standarddeveloped within the 3GPP to improve the Universal MobileTelecommunication System (UMTS) standard to cope with futurerequirements in terms of improved services such as higher data rates,improved efficiency, and lowered costs. The Universal Terrestrial RadioAccess Network (UTRAN) is the radio access network of a UMTS and EvolvedUTRAN (E-UTRAN) is the radio access network of an LTE system. In anUTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to aRadio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS,and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general termfor a radio network node capable of transmitting radio signals to a UEand receiving signals transmitted by a UE.

FIG. 1 illustrates an exemplary wireless communication system. An eNodeB104 serves a UE 106. The eNodeB 104 transmits Downlink (DL)transmissions to the UE 106 and the UE 106 transmits Uplink (UL)transmissions to the eNodeB 104.

Wireless communication systems such as LTE systems can be configured forboth Time Division Duplex (TDD) operation and Frequency Division Duplex(FDD) operation. In TDD systems, the base stations transmit and receiveon the same carrier frequency. UL and DL transmissions are separated intime by designating subframes as either UL subframes or DL subframes. InFDD systems, separate carrier frequencies are used for UL and DLtransmissions.

Typically, a transmitted signal in a radio communication system isorganized in some form of frame structure, or frame configuration. Forexample, LTE generally uses ten equally sized subframes 0-9 of length 1ms per radio frame. In case of TDD, there is generally only a singlecarrier frequency, and UL and DL transmissions are separated in time.Because the same carrier frequency is used for UL and DL transmission,both the base station and the UEs need to switch from transmission toreception and vice versa. An important aspect of a TDD system is toprovide a sufficiently large guard time where neither DL nor ULtransmissions occur in order to avoid interference between UL and DLtransmissions. For LTE, special subframes provide this guard time. A TDDspecial subframe is generally split into three parts: a DL part (DwPTS),a guard period (GP), and an UL part (UpPTS). The remaining subframes areallocated either to UL or DL transmission.

There are seven different TDD UL/DL resource allocations in LTE,illustrated in FIG. 2a . Usually a TDD UL/DL configuration providesabout 40%-90% resources for DL. In the current LTE specification, theUL/DL configuration in a TDD system is semi-statically configured whichmeans that it is not reconfigured so often. As a result, the UL/DLconfiguration sometimes does not match the instantaneous trafficdemands.

It is envisioned that wireless data traffic will become more and morelocalized in the future, as most users tend to gather in the so-calledhotspots, or in indoor areas, or in residential areas. Often, when usersare located in clusters, they tend to generate different UL and DLtraffic patterns at different times. As such, a dynamic feature thatadjusts the UL and DL resource allocations to instantaneous or shortterm traffic variations may be needed in local area cells. Faster TDDreconfigurations, hereinafter referred to as dynamic TDD, have shownpotential for achieving good performance in both UL and DL, especiallyat low to medium system load. Dynamic TDD may become a standardizedfeature in LTE Rel-12. Dynamic TDD systems use the same TDD framestructures as the ones illustrated in FIG. 2a , but allow the TDDconfiguration to be changed depending on current traffic demands.

Different signaling methods that support dynamic TDD reconfigurationswith different time scale are currently being considered. One possibleTDD reconfiguration is allocating each subframe as either UL or DL.However, this option poses challenges to operations such as DL/ULswitching, random access, radio link monitoring, and handover. Moreover,this option also makes it impossible to maintain backward compatibilitywith legacy UEs. A more practical solution is to designate a subset ofsubframes for dynamic TDD reconfiguration. In this case, the subframescan be divided into two types: static subframes and flexible subframes.The static subframes have fixed link directions, UL or DL, whileflexible subframes can be dynamically assigned as either UL or DL.

When dynamic TDD is configured, in general, there are two TDD UL/DLreference configurations, one for UL and one for DL. The TDD ULreference configuration is broadcasted in System Information Block 1(SIB1) and will be used for legacy UEs. Based on the two TDD referenceconfigurations, some subframes may be used as flexible subframes whereeither DL or UL can be configured.

One area of concern with dynamic TDD is Hybrid Automatic Repeat Request(HARQ) timing. A HARQ feedback timing is associated with each DLsubframe. The association determines when to transmit HARQ feedback fora transmission received in the DL subframe. The association is TDDconfiguration dependent.

Furthermore, with flexible subframes it may be difficult for a UE todetermine when to monitor DL control channels and when to perform DL CSImeasurements. A UE may decide to monitor every flexible subframe thathas not been designated for UL transmissions. This may turn out to beunnecessary and would lead to heavy power consumption and falsedetection of a non-existing assignment.

SUMMARY

It is therefore an object to address some of the problems outlinedabove, and to provide a solution enabling a UE operating in a dynamicTDD system applying flexible subframes to determine when to receive DLsignals and when to signal HARQ information. This object and others areachieved by the methods, the network node, and the UE according to theindependent claims, and by the embodiments according to the dependentclaims.

In accordance with a first aspect, a method for enabling a UE todetermine a subframe in which to receive a DL signal and a subframe inwhich to signal HARQ information is provided. The method is performed bya network node of a wireless communication system serving the UE. Thenetwork node is applying dynamic TDD where at least one subframe is aflexible subframe assigned as either a DL subframe or an UL subframe.The method comprises transmitting a first configuration message to theUE indicating a TDD reference configuration enabling the UE to determinethe subframe in which to signal HARQ information. The method furthercomprises transmitting a second configuration message to the UEindicating a set of DL subframes that may comprise explicit signalingmessages. The transmission of the second configuration message enablesthe UE to monitor the indicated set of DL subframes for explicitsignaling messages. The method also comprises transmitting an explicitsignaling message in one of the indicated DL subframes, wherein theexplicit signaling message designates a subframe in which the UE shallreceive the DL signal.

In accordance with a second aspect, a method for determining a subframein which to receive a DL signal and a subframe in which to signal HARQinformation, is provided. The method is performed by a UE served by anetwork node of a wireless communication system. The network node isapplying dynamic TDD where at least one subframe is a flexible subframeassigned as either a DL subframe or an UL subframe. The method comprisesreceiving a first configuration message from the network node indicatinga TDD reference configuration, and determining in which subframe tosignal HARQ information based on the TDD reference configuration. Themethod further comprises receiving a second configuration message fromthe network node indicating a set of DL subframes that may compriseexplicit signaling messages, monitoring the indicated set of DLsubframes for explicit signaling messages, and receiving an explicitsignaling message in response to monitoring the indicated set of DLsubframes. The explicit signaling message designates a subframe in whichthe UE shall receive a DL signal. The method also comprises preparing toreceive the DL signal in the designated subframe.

In accordance with a third aspect, a network node for a wirelesscommunication system configured to serve a UE is provided. The networknode is configured to enable the UE to determine a subframe in which toreceive a DL signal and a subframe in which to signal HARQ information.The network node is further configured to apply dynamic TDD where atleast one subframe is a flexible subframe assigned as either a DLsubframe or an UL subframe. The network node is configured to transmit afirst configuration message to the UE indicating a TDD referenceconfiguration enabling the UE to determine the subframe in which tosignal HARQ information. The network node is further configured totransmit a second configuration message to the UE indicating a set of DLsubframes that may comprise explicit signaling messages, enabling the UEto monitor the indicated set of DL subframes for explicit signalingmessages. The network node is also configured to transmit an explicitsignaling message in one of the indicated DL subframes, wherein theexplicit signaling message designates a subframe in which the UE shallreceive the DL signal.

In accordance with a fourth aspect, a UE for determining a subframe inwhich to receive a DL signal and a subframe in which to signal HARQinformation, is provided. The UE is configured to be served by a networknode of a wireless communication system. The network node is applyingdynamic TDD where at least one subframe is a flexible subframe assignedas either a DL subframe or an UL subframe. The UE is further configuredto receive a first configuration message from the network nodeindicating a TDD reference configuration, and to determine in whichsubframe to signal HARQ information based on the TDD referenceconfiguration. The UE is also configured to receive a secondconfiguration message from the network node indicating a set of DLsubframes that may comprise explicit signaling messages, to monitor theindicated set of DL subframes for explicit signaling messages, and toreceive an explicit signaling message in response to monitoring theindicated set of DL subframes. The explicit signaling message designatesa subframe in which the UE shall receive a DL signal. The UE is alsoconfigured to prepare to receive the DL signal in the designatedsubframe.

An advantage of embodiments is that a semi-static TDD referenceconfiguration is used to determine the HARQ signaling timing, whiledynamic explicit signaling is used to determine subframes in which theUE shall receive a DL signal. The semi-static TDD referenceconfiguration allows for a reliable HARQ procedure. One drawback ofexplicit signaling is that the UE may fail to decode the explicitlysignaled information, which would be a major disadvantage as the HARQsignaling is so important for the system performance. If HARQinformation is not signaled by the UE when expected by the eNodeB, theeNodeB will not be able to perform retransmissions in a correct way.However, the dynamic signaling reduces the need for blind decoding of DLcontrol channels, thus reducing the risk of false detection ofscheduling information as well as reducing the UE power consumption.Furthermore, the dynamic signaling makes it possible to make morereliable Channel State Information (CSI) measurements, as additional CSImeasurement occasions in the subframes designated by the explicitsignaling message are provided.

Advantages of embodiments are thus to allow a UE in a dynamic TDD systemto perform accurate measurements, reduce power consumption, and increasesystem reliability.

Other objects, advantages and features of embodiments will be explainedin the following detailed description when considered in conjunctionwith the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary wirelesscommunications system.

FIG. 2a is a schematic illustration of available TDD configurations forLTE systems.

FIG. 2b is a schematic illustration of an exemplary configuration ofexplicit signaling in subframes.

FIG. 3 is a flow chart illustrating an exemplary method for configuringa UE to monitor explicit signaling.

FIG. 4 is a flow chart illustrating an exemplary method for signalingCSI measurement subframes to a UE using explicit signaling.

FIG. 5 is a flow chart illustrating an exemplary method for configuringa UE to monitor a control channel in a set of subframes using explicitsignaling.

FIGS. 6a-b are block diagrams schematically illustrating embodiments ofthe network node and the UE.

FIG. 7a illustrates an exemplary network node configured to support theexplicit signaling methods disclosed herein.

FIG. 7b illustrates an exemplary wireless device configured to supportthe explicit signaling methods disclosed herein.

FIGS. 8a-b are flowcharts illustrating the method in the network nodeaccording to embodiments.

FIGS. 9a-b are flowcharts illustrating the method in the UE according toembodiments.

DETAILED DESCRIPTION

In the following, different aspects will be described in more detailwith references to certain embodiments and to accompanying drawings. Forpurposes of explanation and not limitation, specific details are setforth, such as particular scenarios and techniques, in order to providea thorough understanding of the different embodiments. However, otherembodiments that depart from these specific details may also exist.

Embodiments are described in a non-limiting general context in relationto an example scenario in an LTE radio network. However, it should benoted that the embodiments may be applied to any network technologyapplying dynamic TDD with flexible subframes.

In a LTE dynamic TDD system such as the one illustrated in FIG. 1, aradio resource such as a channel or a carrier can be dynamicallyconfigured as either UL or DL resource. An implicit Layer 1 (L1)signaling can be used to support dynamic TDD. The link direction (UL orDL) of a flexible subframe may be controlled by the eNodeB 104. ULscheduling grants and DL scheduling assignments are sent to the UE. TheUL grants and the DL assignments implicitly inform the UE of the linkdirection of the subframes concerned by the UL grant or DL assignment.When a UE receives an UL grant concerning a particular subframe, the UEknows that the particular subframe indicated by the UL grant is used forUL transmission. When a UE does not receive an UL grant for a particularsubframe, this implicitly means that the UE shall treat that subframe asDL and monitor the DL control channel of that subframe. In LTE, the DLcontrol channel is referred to as a Physical Downlink Control Channel(PDCCH). An evolved version of the PDCCH is referred to as the enhancedPDCCH (ePDCCH). The UE thus monitors the (e)PDCCH regardless of whetherthis subframe is actually assigned as UL or DL by the eNodeB 104. Thisleads to the following two problems.

First, the UE's power consumption is increased because the UE, withoutan UL grant, have to blindly decode (e)PDCCH in the flexible subframes.When the flexible subframe is scheduled as DL, the UE behavior isacceptable since it has to monitor (e)PDCCH anyway to see whether it isscheduled with DL transmissions in the corresponding subframe. However,when the flexible subframe is scheduled as UL, power is wasted sincethere will be no DL assignment or UL scheduling grant in this subframe.Moreover, there is a risk of a false detection by the UE in the flexiblesubframe, meaning that the UE may falsely detect a non-existing (e)PDCCHDL assignment.

Second, without information about the link direction, it is difficult todo accurate CSI measurements in flexible subframes. One way is to do CSImeasurements no matter if the flexible subframe is scheduled as UL orDL. However, this poses constraints on the UE to not do interferencefiltering in time, since the CSI measurement in UL flexible subframescomprises undesired intra-cell interference which does not exist whenthe flexible subframe is scheduled as a DL subframe. Another way is toonly perform CSI measurement when the UE is scheduled with DLtransmissions, i.e. in subframes known to be DL subframes. Thedisadvantage of this method is that the UE loses a lot of CSImeasurement opportunities, which affects the accuracy of the CSImeasurement.

Another way of solving the signaling of dynamic UL/DL TDD configurationscompared to using the implicit L1 signaling described above is toexplicitly signal a new configuration when it is applied. However, thereis always a risk that a UE may not be able to decode the explicitsignaling message. If the explicit signaling of a new configuration isnot received by the UE, the HARQ procedure will be affected and the UEwill signal HARQ information in an incorrect way. This severely affectsthe system performance and should be avoided.

In embodiments of the invention, the problems described above related tothe implicit as well as the explicit signaling of UL/DL configurationinformation in dynamic TDD systems are addressed by a solution whereexplicit signaling is used, but only for informing the UE aboutsubframes in which the UE shall receive a downlink signal, such as a DLsignal used for measuring CSI or a DL signal comprising a controlchannel. The semi-statically configured UL/DL reference configuration isused for determining in what UL subframe to send the HARQ feedbackinformation related to a transmission received in a DL subframe. Theeffect is that CSI measurement accuracy is improved as well asDiscontinuous reception (DRX) configuration, while at the same timekeeping a robust HARQ procedure.

In some embodiments, a UE is configured to monitor explicit signalingfrom an eNodeB. A UE may be configured by an upper-layer to monitorexplicit signaling messages, signals, or data in a set of predefinedsubframes. These predefined subframes may also be referred to asexplicit signaling monitoring occasions. The predefined subframes may besignaled or sent in a command to the UE. In one embodiment, thepredefined subframes are conveyed to the UE using a set of parameterssuch as a subframe offset and a periodicity of the subframes.

In some embodiments, information transmitted in explicit signalingincludes a set of designated subframes in which the UE should performCSI measurements. In one embodiment, the UE does not perform CSImeasurements in subframes other than those indicated in the explicitsignaling. In another embodiment, the UE may still perform CSImeasurements in subframes other than those indicated in the explicitsignaling.

In some embodiments, explicit signaling is utilized to transmitinformation from an eNodeB to a UE that can be used by the UE for a DRXconfiguration. Information transmitted in explicit signaling may includea set of designated subframes during which the UE is supposed to monitora control channel, e.g., PDCCH or ePDCCH. In subframes other than thoseindicated in the explicit signaling, the UE may enter a DRX inactivemode. During the DRX inactive mode, a UE is not operational and does notmonitor PDCCHs or ePDCCHs.

In some embodiments, a reference configuration, i.e. a TDD UL/DLconfiguration such as one of the TDD configurations illustrated in FIG.2a , is used to inform a UE of the subframes that contain schedulinginformation and HARQ timing information. As stated earlier two TDD UL/DLreference configurations are applied for dynamic TDD, one for UL and onefor DL. Any subframes in addition to those indicated as DL subframes inthe TDD UL/DL reference configuration for UL that the UE is required tomonitor for scheduling information or to perform CSI measurements aresignaled to the UE using explicit signaling. The reference configurationis thus used by the UE to determine when to send HARQ informationrelated to a transmission received in a DL subframe, but the explicitsignaling message indicates DL subframes in which the UE should decodethe control channel and/or measure CSI.

In the present disclosure, methods and apparatuses are disclosed toimprove the CSI measurements and the DRX configuration at a UE side withthe help of explicit signaling. An explicit signaling message, data, orsignal is transmitted from an eNodeB to a UE to help the UE to do CSImeasurement and to monitor (e)PDCCH during a given time period. Thesignaling also ensures that an error caused by a missed signalingmessage is not propagated and that the impact of false detection, i.e.,mistaking one message for a different one, is minimized.

In one embodiment, a UE is configured by higher-layers to monitor anexplicit signaling in a set of predefined subframes. The predefinedsubframes could be signaled by a set of parameters such as a periodicityof the subframes and a subframe offset. The subframes in which the UEshould monitor the explicit signaling satisfy a given conditionspecified by the periodicity and subframe offset, for example as shownin FIG. 2 b.

In FIG. 2b , the dark subframes are explicit signaling monitoringoccasions 200. They are predefined subframes that a UE should monitorfor explicit signaling. The UE may be configured to monitor the explicitsignaling in fixed DL subframes, e.g. subframe 0 in every radio frame.In one embodiment, the configuration is signaled by higher layers. Insome embodiments, a separate Radio Network Temporary Identifier (RNTI)is also configured to be used for detection of configuration messages.The network may configure multiple users with the same RNTI, bymulti-casting the configuration message. In another embodiment, thenetwork may configure different RNTIs to different users by uni-castingthe configuration message. The already assigned Cell RNTI (C-RNTI),different for different user, can be used for this purpose.

In one embodiment, the explicit signaling is sent on the (e)PDCCH in aDownlink Control Information (DCI) either by reusing bits in one ofexisting DCIs or by constructing a new DCI. In some embodiments, the DCIis sent in the common search space on the control channel. In otherembodiments the explicit signaling may be sent in the common searchspace or in a UE specific search space. The signaling may be also sentin a Medium Access Control (MAC) control element.

In another embodiment, a new physical layer signal is introduced for theexplicit signaling. The signal may, for example, be carried in theresource elements in the PDCCH region not mapped to any of the existingchannels. In yet another embodiment the signal is mapped to resourceelements coverable by one or multiple CSI-Reference Signalconfigurations.

In some embodiments, the new explicit configuration, i.e. the subframesthat are designated in the explicit signaling message received during anexplicit signaling monitoring occasion, is applied a fixed time afterthe monitoring occasion. The explicitly signaled configuration is thusreceived by the UE at the monitoring occasion, but the configuration isnot to be applied until a fixed time has lapsed from the monitoringoccasion.

In some embodiments, the UE does not need to monitor the explicitsignaling if the UE is in a DRX inactive mode at the time instance ofthe monitoring occasion. In some embodiments, the UE can be configurednot to monitor the explicit signaling at all.

In a further embodiment, the content of the explicit signaling messagecomprises information regarding the subframes in which the UE canperform CSI measurements and configure DRX. In one example, the explicitsignaling is defined as bitmap or as an UL/DL TDD configuration. The UEperforms CSI measurements and monitors (e)PDCCH in subframes that areindicated as DL subframes.

Explicit Signaling to Support CSI Measurements

Explicit signaling can be used to improve CSI measurements. In oneembodiment, upon receiving the explicit signaling, the UE performs CSImeasurements only in a given set of subframes indicated by the explicitsignaling. In subframes other than those indicated by the explicitsignaling, the UE does not perform CSI measurements. In a furtherembodiment, the CSI reference resource is given as a subset of thesubframes indicated by the explicit signaling. As an example, in dynamicTDD, the explicit signaling signals an UL/DL reference configuration,and the UE shall only do CSI measurements in subframes that areindicated as DL subframes.

In a further embodiment, when the explicit signaling cannot be detectedby the UE at an explicit signaling monitoring occasion, the UE avoidsperforming CSI measurements in all subframes during the interval betweentwo consecutive explicit signaling monitoring occasions. In anotherembodiment, when the explicit signaling cannot be detected by the UE atan explicit signaling monitoring occasion, the UE performs CSImeasurements in subframes when it is scheduled for DL transmissionsduring the interval between two consecutive explicit signalingmonitoring occasions. The reasoning is that the UE knows that thesubframe is a DL subframe when it receives a DL assignment in thesubframe. In yet another embodiment, the UE performs CSI measurementsaccording to a fallback or default configuration if the explicitsignaling is not detected. The fallback or default configuration maye.g. be the TDD UL/DL configuration for UL.

Explicit Signaling to Support DRX Configuration and PDCCH Monitoring

Explicit signaling can be used to improve DRX. In one embodiment, uponreceiving an explicit signaling, the UE monitors (e)PDCCH only in agiven set of subframes indicated by the explicit signaling. In someembodiments, the UE only monitors those subframes indicated by theexplicit signal, i.e. designated subframes in the received explicitsignaling message, and shall not monitor PDCCH/ePDCCH in other subframesto avoid false detection. In other embodiments, the UE monitors thosesubframes indicated by the explicit signaling. The UE may decide not tomonitor subframes other than those signaled by the explicit signaling tosave battery power. But the UE may also decide to monitor (e)PDCCH insubframes other than those indicated by the explicit signaling.

In subframes where the UE does not monitor PDCCH/ePDCCH, the UE can turnoff the radio front end or the base band, or both in all or in a subsetof the signaled subframes to save energy or processing resources. As anexample, in dynamic TDD, the explicit signaling signals an UL/DLreference configuration, and the UE monitors (e)PDCCH in subframes thatare indicated as DL subframes according to that UL/DL referenceconfiguration. In other subframes, the UE enters a DRX inactive mode tosave battery power. This embodiment is not limited to the scope ofdynamic TDD but may also apply for other use cases. An example ofanother use case is when the explicit signaling may be applied in anetwork configured with almost blank subframes (ABS) in for example anFDD LTE system, which leaves a number of subframes unscheduled in anode. Terminals served by such node can be informed about subframeswhere (e)PDCCH should not or does not need to be monitored.

In a further embodiment, when the explicit signaling cannot be detectedby the UE at an explicit signaling monitoring occasion, the UE avoidsmonitoring (e)PDCCH in all subframes that may be (e)PDCCH subframesduring the interval between two consecutive explicit signalingmonitoring occasions. In one embodiment the UE follows a fallback ordefault configuration if the explicit signaling is not detected. Thedefault configuration can be configured by higher layers, e.g.semi-statically configured, or specified in the standard. In anotherembodiment, when the explicit signaling is not detected by the UE at anexplicit signaling monitoring occasion, the UE may monitor (e)PDCCH inall applicable subframes that may possibly contain (e)PDCCH according toany of the signaling options and that are not scheduled for ULtransmissions, i.e. in subframes in which the UE is not scheduled for ULtransmissions, during the interval between two consecutive explicitsignaling monitoring occasions.

HARQ Signaling

Another application of the explicit signaling methods disclosed hereinis HARQ and scheduling timing. In one embodiment, scheduling and HARQtiming is determined by a reference configuration, such as a TDD UL/DLconfiguration, and the explicit signaling is used to signal additionalsubframes to be monitored for DL scheduling and CSI measurements. Asexplained in the background section, there are in general two TDD UL/DLreference configurations, one for UL and one for DL, when dynamic TDD isconfigured. The additional subframes to be monitored for DL schedulingand CSI measurements mentioned above refers to subframes in addition toDL subframes defined by the TDD configuration for UL. The subframes area subset of DL subframes in the TDD configuration for DL. The format ofthe DL control channel messages, e.g. the interpretation ofUL-index/downlink assignment index bits in an UL grant, can then alsofollow a default configuration.

In one embodiment, UL scheduling timing is based on the referenceconfiguration in subframes where the reference configuration has DLsubframes. In other subframes, UL scheduling timing follow a TDDconfiguration according to a configuration sent in the explicitsignaling message.

Methods and Apparatus

FIG. 3 is an exemplary flow chart illustrating a method of configuringexplicit signaling at a UE. In FIG. 3, the UE receives a signal, forexample, an upper-level command, indicating a set of pre-defined DLsubframes (step 302). The UE then monitors the set of pre-definedsubframes for explicit signaling on a DL channel (step 304).

FIG. 4 is an exemplary flow chart illustrating a method of configuringCSI measurements at a UE using explicit signaling. After being informedof the monitoring occasions for explicit signaling, the UE monitors theset of pre-defined DL subframes for explicit signalingmessages/data/signals (step 402). The UE receives an explicit signalingmessage/data/signal that indicates a set of designated subframes (step404). The UE performs CSI measurements during one or more subframesincluded in the set (step 406).

FIG. 5 is an exemplary flow chart illustrating a method of configuring aUE to monitor a control channel in a set of designated subframes usingexplicit signaling. After being informed of the monitoring occasions forexplicit signaling, the UE monitors the set of pre-defined DL subframesfor explicit signaling messages/data/signals (step 502). The UE receivesan explicit signaling message/data/signal that indicates a set ofdesignated subframes (step 504). During each designated subframe, the UEmonitors a control channel, e.g., PDCCH/ePDCCH. In one or more subframesthat are not included in the set, the UE selectively enters a DRXinactive mode and stops monitoring the control channel.

FIG. 8a is a flowchart illustrating one embodiment of a method forenabling a UE 700 to determine a subframe in which to receive a DLsignal and a subframe in which to signal or transmit HARQ information.The method is performed by a network node 600 of a wirelesscommunication system serving the UE. The network node is applyingdynamic TDD, where at least one subframe is a flexible subframe assignedas either a DL subframe or an UL subframe. The method comprises:

800: Transmitting a first configuration message to the UE indicating aTDD reference configuration, i.e. a reference configuration used for TDDsuch as one of the configurations illustrated in FIG. 2a , enabling theUE to determine subframes in which to signal HARQ information. The firstconfiguration message may be transmitted by higher layers. The firstconfiguration message may be a configuration message thatsemi-statically configures the UE with a TDD reference configuration.The TDD reference configuration is thus not changed so often.

810: Transmitting a second configuration message to the UE indicating aset of DL subframes that may comprise explicit signaling messages,enabling the UE to monitor the indicated set of DL subframes forexplicit signaling messages. The set of DL subframes that may compriseexplicit signaling messages may be indicated by a subframe offset and aperiodicity, as illustrated in FIG. 2b . The second configurationmessage may be transmitted together with the first configuration messagein one message by a higher layer, or it may be sent in a messageseparate from the first configuration message. The second configurationmessage indicating the set of DL subframes may comprise an RNTIassociated with the UE, such that the UE can identify whether the secondconfiguration message is directed to the UE. The network may configuremultiple users with the same RNTI, by multi-casting the configurationmessage. In another embodiment, the network may configure differentRNTIs to different users by uni-casting the configuration message. TheC-RNTI which is different for different UEs can be used for thispurpose.

820: Transmitting an explicit signaling message in one of the indicatedDL subframes, wherein the explicit signaling message designates asubframe in which the UE shall receive the DL signal. The explicitsignaling message may e.g. comprise an indicator of one of the TDDconfigurations illustrated in FIG. 2a , thus indicating to the UEreceiving the signaling message that the indicated TDD configurationdesignates the DL subframes in which to receive the DL signal. In oneembodiment, the DL signal is to be used by the UE to measure CSI. Inanother embodiment, which may be combined with the previous embodiment,the DL signal comprises a DL control channel to be received by the UE,such as PDCCH or ePDCCH. In embodiments, the designation in the explicitsignaling message may be applied a fixed time after the transmission ofthe explicit signaling message. If, as an example, the explicitsignaling takes place in a radio frame, the explicitly signaledconfiguration may not be applied until a subsequent radio frame. In oneembodiment, the wireless communication system is an LTE system and theexplicit signaling message may be transmitted in a common search spaceon the (e)PDCCH. The explicit signaling message may be transmitted onthe (e)PDCCH in a DCI either by reusing bits in one of existing DCIs orby constructing a new DCI.

FIG. 8b is a flowchart illustrating another embodiment of the method.The method optionally also comprises after the transmitting in 800, 810,and 820 described with reference to FIG. 8a above:

830: Transmitting the DL signal in the designated subframe.

FIG. 9a is a flowchart illustrating one embodiment of a method fordetermining a subframe in which to receive a DL signal and a subframe inwhich to signal HARQ information. The method is performed by the UE 700served by the network node 600 of a wireless communication system. Thenetwork node is applying dynamic TDD, where at least one subframe is aflexible subframe assigned as either a DL subframe or an UL subframe.The method comprises:

900: Receiving a first configuration message from the network nodeindicating a TDD reference configuration. The first configurationmessage may be a configuration message that semi-statically configuresthe UE with a TDD reference configuration. The TDD referenceconfiguration is thus not changed so often.

910: Determining in which subframe to signal HARQ information based onthe TDD reference configuration. By using a TDD reference configurationfor the UL/DL TDD configuration to determine when to signal HARQ in theUL, the HARQ procedure is consistent and reliable HARQ information isprovided which is important for the system performance.

920: Receiving a second configuration message from the network nodeindicating a set of DL subframes that may comprise explicit signalingmessages. The set of DL subframes that may comprise explicit signalingmessages may be indicated by a subframe offset and a periodicity. In oneexample the explicit signaling may be indicated to take place insubframe 0 of every radio frame. The second configuration messageindicating the set of DL subframes may comprise an RNTI, and the secondconfiguration message may be received by the UE when the RNTI isassociated with the UE.

930: Monitoring the indicated set of DL subframes for explicit signalingmessages. The indicated set of DL subframes may be monitored forexplicit signaling messages only when the UE is in a DRX active mode,i.e., when the UE is operational and monitors the (e)PDCCH.

940: Receiving an explicit signaling message in response to or as aresult of monitoring the indicated set of DL subframes. The explicitsignaling message designates a subframe in which the UE shall receive aDL signal. In one embodiment, the wireless communication system is anLTE system and the explicit signaling message is received in a commonsearch space of a PDCCH. The designation in the explicit signalingmessage may be applied a fixed time after receiving the explicitsignaling message, as already described above in step 820 of the methodin the network node. An advantage of the explicit signaling message isthat the UE may use the information given in the explicit signalingmessage to determine more subframes for doing e.g. CSI measurements thanwould be possible with information only regarding the semi-staticallyconfigured TDD reference configuration. The CSI measurements maytherefore be more reliable as more CSI measurement possibilities areprovided. With only a semi-statically configured TDD referenceconfiguration, there will be subframes for which the UE cannot determinea link direction, and for which the UE should thus avoid doing e.g. CSImeasurements.

950: Preparing to receive the DL signal in the designated subframe. Inembodiments, the DL signal is used to measure CSI. The DL signal mayalso comprise a DL control channel, such as (e)PDCCH.

The method may further comprise to receive the DL signal in a subframeaccording to a fallback or default configuration until the next occasionfor monitoring for explicit signaling messages, when no explicitsignaling message is received at the monitoring 930 of the indicated setof DL subframes. In this example embodiment, if the UE fails to decodethe explicit signaling message, i.e. when no explicit signaling messageis received in response to monitoring the indicated set of DL subframes,the UE may use a fallback configuration to determine when to receive theDL signal. This fallback configuration is used until next monitoringoccasion, when the UE may be able to decode the explicit signalingmessage and act accordingly. Alternatively, the UE may use the TDDreference configuration received in the first configuration message todetermine when to receive the DL signal.

FIG. 9b is a flowchart illustrating another embodiment of the method.The method optionally also comprises after the steps in 900-950described with reference to FIG. 9a above:

960: Receiving the DL signal in the designated subframe.

970: Turning off the radio front end and/or the base band processing inone or more subframes other than the designated subframe.

An embodiment of the network node 600 for a wireless communicationnetwork is schematically illustrated in the block diagram in FIG. 6a .The network node 600 is configured to serve a UE 700, and to enable theUE to determine a subframe in which to receive a DL signal and asubframe in which to signal HARQ information. The network node isfurther configured to apply dynamic TDD where at least one subframe is aflexible subframe assigned as either a DL subframe or an UL subframe.The network node is configured to transmit a first configuration messageto the UE indicating a TDD reference configuration enabling the UE todetermine the subframe in which to signal HARQ information. The firstconfiguration message may be a configuration message thatsemi-statically configures the UE with a TDD reference configuration.The TDD reference configuration is thus not changed so often. Thenetwork node is also configured to transmit a second configurationmessage to the UE indicating a set of DL subframes that may compriseexplicit signaling messages, enabling the UE to monitor the indicatedset of DL subframes for explicit signaling messages. The network node isfurther configured to transmit an explicit signaling message in one ofthe indicated DL subframes, wherein the explicit signaling messagedesignates a subframe in which the UE shall receive the DL signal.

In embodiments, the network node is further configured to transmit theDL signal in the designated subframe.

The DL signal is in embodiments to be used by the UE to measure CSI. Thereceived signal may comprise a DL control channel to be received by theUE.

In embodiments, the set of DL subframes that may comprise explicitsignaling messages may be indicated by a subframe offset and aperiodicity. The second configuration message indicating the set of DLsubframes may comprise an RNTI associated with the UE, such that the UEcan identify whether the second configuration message is directed to theUE. The network may configure multiple users with the same RNTI, bymulti-casting the configuration message. In another embodiment, thenetwork may configure different RNTIs to different users by uni-castingthe configuration message. The C-RNTI which is different for differentUEs can be used for this purpose.

In one embodiment, the wireless communication system is an LTE system,and the network node 600 is configured to transmit the explicitsignaling message in a common search space on a PDCCH.

In embodiments, the network node 600 may be further configured to applythe designation in the explicit signaling message a fixed time after thetransmission of the explicit signaling message.

An embodiment of the UE 700 is also schematically illustrated in theblock diagram in FIG. 6a . The UE 700 is configured to determine asubframe in which to receive a DL signal and a subframe in which tosignal HARQ information. The UE is also configured to be served by anetwork node of a wireless communication system. The network node isconfigured to apply dynamic TDD where at least one subframe is aflexible subframe assigned as either a DL subframe or an UL subframe.The UE 700 is further configured to receive a first configurationmessage from the network node indicating a TDD reference configuration,and to determine in which subframe to signal HARQ information based onthe TDD reference configuration. The first configuration message may bea configuration message that semi-statically configures the UE with aTDD reference configuration. The TDD reference configuration is thus notchanged so often. The UE 700 is also configured to receive a secondconfiguration message from the network node indicating a set of DLsubframes that may comprise explicit signaling messages, monitor theindicated set of DL subframes for explicit signaling messages, andreceive an explicit signaling message in response to monitoring theindicated set of DL subframes. The explicit signaling message designatesa subframe in which the UE shall receive a DL signal. The UE 700 isfurther configured to prepare to receive the DL signal in the designatedsubframe.

In embodiments, the UE 700 is further configured to receive the DLsignal in the designated subframe. The DL signal may be used to measureCSI. The DL signal may also comprise a DL control channel.

The UE 700 may in embodiments be further configured to turn off at leastone of the radio front end and the base band processing in one or moresubframes other than the designated subframe.

The set of DL subframes that may comprise explicit signaling messagesmay be indicated by a subframe offset and a periodicity. The secondconfiguration message indicating the set of DL subframes may comprise anRNTI, and the UE may be further configured to receive the secondconfiguration message when the RNTI is associated with the UE.

In one embodiment, the wireless communication system is an LTE system,and the UE is further configured to receive the explicit signalingmessage in a common search space of a PDCCH or ePDCCH.

In embodiments, the designation in the explicit signaling message may beapplied a fixed time after receiving the explicit signaling message.

In one embodiment, the UE 700 is further configured to monitor theindicated set of DL subframes for explicit signaling messages only whenthe UE is in a DRX active mode.

The UE 700 may be further configured to receive the DL signal in asubframe according to a fallback configuration until the next occasionfor monitoring for explicit signaling messages. This may be done when noexplicit signaling message is received at the monitoring of theindicated set of DL subframes.

In embodiments of the invention, the network node 600 may comprise aprocessor 622 and a memory 623. The network node 600 may also comprise atransmitter 620 and a receiver 621 configured to communicate with the UE700, and connected to the processor 622. One or more antennas 602 areconnected to the transmitter 620 and the receiver 621. The memory 623may comprise instructions executable by the processor 622. The networknode 600 may thereby be operative to transmit a first configurationmessage to the user equipment indicating a TDD reference configurationenabling the user equipment to determine subframes in which to signalHARQ information. The network node 600 may also be operative to transmita second configuration message to the user equipment indicating a set ofdownlink subframes that may comprise explicit signaling messages,enabling the user equipment to monitor the indicated set of downlinksubframes for explicit signaling messages. The network node 600 mayfurther be operative to transmit an explicit signaling message in one ofthe indicated downlink subframes, wherein the explicit signaling messagedesignates a subframe in which the user equipment shall receive thedownlink signal.

In embodiments of the invention, the UE 700 may comprise a processor 722and a memory 723. The UE 700 may also comprise a transmitter 720 and areceiver 721 configured to communicate with the network node 600, andconnected to the processor 722. One or more antennas 706 are connectedto the transmitter 720 and the receiver 721. The memory 723 may compriseinstructions executable by the processor 722. The UE 700 may thereby beoperative to receive a first configuration message from the network nodeindicating a TDD reference configuration, and to determine in whichsubframe to signal HARQ information based on the TDD referenceconfiguration. The UE 700 may also be operative to receive a secondconfiguration message from the network node indicating a set of downlinksubframes that may comprise explicit signaling messages, to monitor theindicated set of downlink subframes for explicit signaling messages, andto receive an explicit signaling message in response to monitoring theindicated set of downlink subframes, wherein the explicit signalingmessage designates a subframe in which the user equipment shall receivea downlink signal. The UE 700 may further be operative to prepare toreceive the downlink signal in the designated subframe.

In an alternative way to describe the embodiment in FIG. 6a ,illustrated in FIG. 6b , the network node 600 comprises a firsttransmitting module 630 adapted to transmit a first configurationmessage to the user equipment indicating a TDD reference configurationenabling the user equipment to determine a subframe in which to signalHARQ information. The network node 600 also comprises a secondtransmitting module 631 adapted to transmit a second configurationmessage to the user equipment indicating a set of downlink subframesthat may comprise explicit signaling messages, enabling the userequipment to monitor the indicated set of downlink subframes forexplicit signaling messages. The network node 600 further comprises athird transmitting module 632 adapted to transmit an explicit signalingmessage in one of the indicated downlink subframes, wherein the explicitsignaling message designates a subframe in which the user equipmentshall receive the downlink signal. The modules described above arefunctional units which may be implemented in hardware, software,firmware or any combination thereof. In one embodiment, the modules areimplemented as a computer program running on a processor.

In FIG. 6b , the UE 700 comprises a first receiving module 731 adaptedto receive a first configuration message from the network nodeindicating a TDD reference configuration. The UE 700 comprises a firstdetermining module 732 adapted to determine in which subframe to signalHARQ information based on the TDD reference configuration, and a secondreceiving module 733 adapted to receive a second configuration messagefrom the network node indicating a set of downlink subframes that maycomprise explicit signaling messages. The UE 700 also comprises amonitoring module 734 adapted to monitor the indicated set of downlinksubframes for explicit signaling messages, and a third receiving module735 adapted to receive an explicit signaling message in response tomonitoring the indicated set of downlink subframes, wherein the explicitsignaling message designates a subframe in which the user equipmentshall receive a downlink signal. The UE 700 further comprises apreparing module 736 adapted to prepare to receive the downlink signalin the designated subframe.

In an alternative way to describe the embodiment in FIG. 6a , thenetwork node 600 and the UE 700 each comprise a Central Processing Unit(CPU) which may be a single unit or a plurality of units. Furthermore,the network node 600 and the UE 700 comprise at least one computerprogram product (CPP) in the form of a non-volatile memory, e.g. anEEPROM (Electrically Erasable Programmable Read-Only Memory), a flashmemory or a disk drive. The CPPs of the network node and the UE comprisea computer program each, which comprises code means which when run onthe network node 600 and the UE 700 respectively causes the CPU toperform steps of the procedure described earlier in conjunction withFIGS. 8a-b and 9a-b . In other words, when said code means are run onthe CPU, they correspond to the processors 622 and 722 of FIG. 6 a.

FIG. 7a illustrates an exemplary network node 600 configured to supportexplicit signaling methods disclosed in the present application. Thenetwork node 600 comprises an antenna system 602, a transceiver 604, andprocessing circuits 606. The antenna system 602 is configured totransmit and receive radio signals. The transceiver 604 is configured toprepare (up-convert, digital-to-analogue convert, etc.) transmit signalsand to process (down-convert and analogue-to-digital convert, etc.)received signals. The processing circuits 606 comprise a TDDconfiguration unit or circuit 608 and an explicit signaling unit orcircuit 610. The TDD configuration unit 608 configures a radio resource(carrier, frequency or channel) for a UE's DL and UL transmissions. Inone embodiment, the TDD configuration unit 608 may allocate certain LTEsubframes for the UE's UL transmissions and allocate certain LTEsubframes for the UE's DL transmissions. In some embodiments, somesubframes may be designated for UL or DL transmissions only. Thosesubframes are referred to as fixed subframes. If a subframe can be usedfor UL and DL transmissions, although not at the same time, the subframeis referred to as a flexible subframe. Also some subframes may bereserved as almost blank subframes (ABS). In some embodiments, thenetwork node may receive UL scheduling grants for certain flexiblesubframes. The UE may treat a flexible subframe that has not beenscheduled for UL as a DL subframe. This is referred to as “implicitsignaling” as compared to the explicit signaling methods describedherein. The UE may perform CSI measurements and control channelmonitoring during such flexible subframes. In some embodiments, the UEis explicitly informed of the subframes for DL transmissions. Theexplicit signaling unit 608 is configured to signal one or more sets ofdesignated subframes to the UE. The UE performs CSI measurements and/orcontrol channel monitoring based on the one or more sets of designatedsubframes. The UE may also configure DRX using an explicitly signaledset of subframes.

FIG. 7b illustrates an exemplary UE 700 configured to support theexplicit signaling methods disclosed herein. The UE comprises atransceiver 702, processing circuits 704, and an antenna system 706. Thetransceiver 702 is configured to transmit and receive radio signals viathe antenna system 706. The processing circuits 704 further comprise anexplicit signaling monitoring circuit 712, a CSI measurement circuit 708and a control channel monitoring circuit 710. The explicit signalingmonitoring circuit 712 is configured to monitor a set of predefinedsubframes for explicit signaling messages/data/signals. In someembodiments, the set of predefined subframes is received via anupper-level or higher-level command. The CSI measurement circuit 708 isconfigured to perform CSI measurements. In some embodiments, the CSImeasurement circuit 708 is configured to perform CSI measurements duringa set of designated subframes. The set of designated subframes may bereceived via explicit signaling from the eNodeB 104. The control channelmonitoring circuit 710 is configured to monitor control channels, e.g.,PDCCH or ePDCCH. In some embodiments, the control channel monitoringcircuit 710 is configured to monitor a control channel during a set ofdesignated subframes. The set of designated subframes may be receivedvia explicit signaling from e.g. the eNodeB 104 or the network node 600.In subframes other than those received via explicit signaling, the UE700 may enter a DRX inactive mode. During the DRX inactive mode, the UEdoes not monitor the control channels. In some embodiments, the UE 700enters a DRX inactive mode in every subframe other than those indicatedin explicit signaling and does not monitor control channels in thosesubframes. Alternatively, the UE 700 may choose to monitor a controlchannel in some of the subframes that are not designated for controlchannel monitoring by explicit signaling and enters a DRX inactive modewhen not monitoring.

The above mentioned and described embodiments are only given as examplesand should not be limiting. Other solutions, uses, objectives, andfunctions within the scope of the accompanying patent claims may bepossible.

The invention claimed is:
 1. A method for enabling a user equipmentserved by a network node to determine a subframe in which to receive adownlink signal, the method being performed by the network node in adynamic Time Division Duplex (TDD) wireless communication system whereat least one subframe is a flexible subframe assigned as either adownlink subframe or an uplink subframe, the method comprising:transmitting a configuration message to the user equipment indicating afixed downlink subframe that comprises an explicit signaling message,wherein the explicit signaling message designates a subframe in whichthe user equipment should receive the downlink signal, and wherein thefixed downlink subframe is designated for downlink transmissions only;and transmitting the explicit signaling message in the indicated fixeddownlink subframe.
 2. The method of claim 1, wherein the configurationmessage indicates a TDD reference configuration.
 3. The method of claim1, further comprising transmitting the downlink signal in the designatedsubframe.
 4. The method of claim 1, wherein the downlink signal is asignal for measuring Channel Status Information (CSI).
 5. The method ofclaim 1, wherein the downlink signal comprises a downlink controlchannel to be received by the user equipment.
 6. The method of claim 1,wherein the configuration message indicates a set of fixed downlinksubframes one of which comprises the explicit signaling message.
 7. Themethod of claim 6, wherein the configuration message indicates the setof fixed downlink subframes using a subframe offset or a periodicity. 8.The method of claim 1, wherein the configuration message indicates aradio network temporary identifier associated with the user equipment.9. The method of claim 1: wherein the wireless communication system is aLong Term Evolution (LTE) system; and wherein the explicit signalingmessage is transmitted in a common search space on a physical downlinkcontrol channel (PDCCH).
 10. The method of claim 1, wherein thedesignation in the explicit signaling message is applied a fixed timeafter the transmission of the explicit signaling message.
 11. A methodfor determining a subframe in which to receive a downlink signal, themethod being performed by a user equipment served by a network node in adynamic Time Division Duplex (TDD) wireless communication system whereat least one subframe is a flexible subframe assigned as either adownlink subframe or an uplink subframe, the method comprising:receiving a configuration message from the network node indicating afixed downlink subframe that comprises an explicit signaling message,wherein the explicit signaling message designates a subframe in whichthe user equipment should receive the downlink signal, and wherein thefixed downlink subframe is designated for downlink transmissions only;and receiving the explicit signaling message in the designated subframe.12. The method of claim 11, wherein the configuration message is a TDDreference configuration.
 13. The method of claim 12, wherein the methodfurther comprises determining in which subframe to signal HARQinformation based on the TDD reference configuration.
 14. The method ofclaim 11, wherein the method further comprises receiving the downlinksignal in the designated subframe.
 15. The method of claim 11, whereinthe method further comprises using the downlink signal to measureChannel Status Information (CSI).
 16. The method of claim 11, whereinthe downlink signal comprises a downlink control channel.
 17. The methodof claim 11, wherein the configuration indicates a set of fixed downlinksubframes one of which comprises the explicit signaling message.
 18. Themethod of claim 17, further comprising turning off at least one of aradio front end and base band processing in subframes other than one ormore designated subframes designated in one or more explicit signalingmessages of the set of fixed downlink subframes.
 19. The method of claim17, wherein the configuration indicates the set of fixed downlinksubframes using a subframe offset or a periodicity.
 20. The method ofclaim 17, wherein the method further comprises monitoring the indicatedset of fixed downlink subframes for the explicit signaling message onlywhen the user equipment is in a discontinuous reception (DRX) activemode.
 21. The method of claim 17, further comprising, when no explicitsignaling message is received when monitoring of the indicated set offixed downlink subframes for the explicit signaling message, receivingthe downlink signal in a subframe according to a fallback configurationuntil a next occasion for monitoring for any explicit signalingmessages.
 22. The method of claim 11: wherein the configuration messagecomprises a radio network temporary identifier; and wherein theconfiguration message is received when the radio network temporaryidentifier is associated with the user equipment.
 23. The method ofclaim 11: wherein the wireless communication system is a Long TermEvolution (LTE) system; and wherein the explicit signaling message isreceived in a common search space of a physical downlink control channel(PDCCH).
 24. The method of claim 11, wherein the designation in theexplicit signaling message is applied a fixed time after receiving theexplicit signaling message.
 25. A network node for enabling a userequipment served by the network node to determine a subframe in which toreceive a downlink signal, the network node comprising: a processor anda memory, the memory containing instructions executable by the processorwhereby the network node is configured to: operate in a dynamic TimeDivision Duplex (TDD) system where at least one subframe of a pluralityof subframes is a flexible subframe assigned as either a downlinksubframe or an uplink; transmit a configuration message to the userequipment indicating of the plurality of subframes a fixed downlinksubframe that comprises an explicit signaling message, wherein theexplicit signaling message designates a subframe in which the userequipment should receive the downlink signal, and wherein the fixeddownlink subframe is designated for downlink transmissions only; andtransmit the explicit signaling message in the indicated fixed downlinksubframe.
 26. A user equipment for determining a subframe in which toreceive a downlink signal, the user equipment comprising: a processorand a memory, the memory containing instructions executable by theprocessor whereby the user equipment is configured to: operate in adynamic Time Division Duplex (TDD) system where at least one subframe ofa plurality of subframes is a flexible subframe assigned as either adownlink subframe or an uplink; receive a configuration message from thenetwork node indicating of the plurality of subframes a fixed downlinksubframe that comprises an explicit signaling message, wherein theexplicit signaling message designates a subframe in which the userequipment should receive the downlink signal, and wherein the fixeddownlink subframe is designated for downlink transmissions only; andreceive the explicit signaling message in the designated subframe.